First, consider that from an optimization standpoint Guardian is considered a very strong (sky blue, if you're familiar with CharOp ratings) theme for melee characters simply for its level 1 power; anything more than that is just icing on the cake.
Next, consider the statement
you’re supposed to protect people who don’t fight as well as you do.
If the warlock can beat the guardian in a 1-on-1 fight, then this probably does not apply to the warlock. Based on that part of it, I personally would probably rule the warlock to not be a valid bonded charge.
On the other hand, it's not much fun to get a new ability that's entirely useless. Are there any other more reasonable choices available? An NPC that travels with the party? A wizard, invoker, seeker, or psion? Maybe even a bard. If yes, then you're probably on safe ground ruling that the warlock is not a valid target, but the other option is.
If there's nobody else who's even remotely valid as a bonded charge, then consider allowing the player to select the warlock as his bonded charge. You may want to warn him that if the bonded charge ability seems too powerful, or the warlock saves him even half as often as he saves the warlock, then you'll break the bond.